linux-kernel - [PATCH 05/10] x86/traps: Clear TIF_BLOCKSTEP on all debug exceptions

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Message-Id: <9b123f7a348490daf1bec2b3f145737c612d7281.1456720860.git.luto@kernel.org>
Date:	Sun, 28 Feb 2016 21:28:50 -0800
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org
Cc:	linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Brian Gerst <brgerst@...il.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 05/10] x86/traps: Clear TIF_BLOCKSTEP on all debug exceptions

The SDM says that debug exceptions clear BTF, and we need to keep
TIF_BLOCKSTEP in sync with BTF.  Clear it unconditionally and improve
the comment.

I suspect that the fact that kmemcheck could cause TIF_BLOCKSTEP not
to be cleared was just an oversight.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/kernel/traps.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index dd2c2e66c2e1..19e6cfa501e3 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -598,6 +598,13 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	dr6 &= ~DR6_RESERVED;
 
 	/*
+	 * The SDM says "The processor clears the BTF flag when it
+	 * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
+	 * TIF_BLOCKSTEP in sync with the hardware BTF flag.
+	 */
+	clear_tsk_thread_flag(tsk, TIF_BLOCKSTEP);
+
+	/*
 	 * If dr6 has no reason to give us about the origin of this trap,
 	 * then it's very likely the result of an icebp/int01 trap.
 	 * User wants a sigtrap for that.
@@ -612,11 +619,6 @@ dotraplinkage void do_debug(struct pt_regs *regs, long error_code)
 	/* DR6 may or may not be cleared by the CPU */
 	set_debugreg(0, 6);
 
-	/*
-	 * The processor cleared BTF, so don't mark that we need it set.
-	 */
-	clear_tsk_thread_flag(tsk, TIF_BLOCKSTEP);
-
 	/* Store the virtualized DR6 value */
 	tsk->thread.debugreg6 = dr6;
 
-- 
2.5.0